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Teflon for Teeth

I may come to regret the title of this post given the chemophobia and fear surrounding fluoride and water fluoridation, but recent research suggests that fluoride may help prevent bacteria from causing cavities by creating a non-stick surface on teeth.

There are several ways that fluoride may prevent cavities. Tooth enamel is made of a mineral called hydroxyapatite that, though strong, can be eroded by acids made by the bacteria in our mouths or contained in our food and drink. Fluoride treatment replaces the hydroxyl group in hydroxyapatite with fluoride ions to form a new mineral called fluorapatite. Conventional thinking is that this replacement strengthens the enamel so that it can better resist an acidic onslaught. There is also evidence that fluoride can slow bacterial metabolism and prevent the formation of biofilms making it harder for bacteria to live on our teeth. More recently, studies have begun to suggest that fluoride may also make it harder for bacteria to adhere to the surface of our teeth.

“Block diagram of atomic force microscope using beam deflection detection. As the cantilever is displaced via its interaction with the surface, so too will the reflection of the laser beam be displaced on the surface of the photodiode.”

Using atomic force microscopy, researchers from Germany set out to measure how strongly cavity-causing bacteria clung to fluoride-treated versus untreated surfaces. This technique makes use of bacteria-coated probes fixed to a cantilever that is lowered to make contact with the the “tooth” surface. The probe is then retracted, which tugs on the bacteria touching the surface. A laser is used to measure the displacement of the cantilever from which the force applied can be calculated.

Performing these experiments on actual teeth, however, is complicated because the surface of teeth and the presence of fluoride vary from tooth to tooth. Instead, the scientists opted to use hydroxapatite pellets as sort of “false teeth.” These pellets have smoother surfaces and the same chemical composition to better control for these variabilities. Fluoride treatment was carried out by soaking the pellets for 5 minutes in a solution containing fluoride at 1000 ppm (parts per million), which is the typical concentration of fluoride in toothpastes. The fluoride gel treatments you might get a dentist’s office can be as high as ~12,000 ppm.

Bacteria stick to fluoride-treated “tooth” surfaces with ~50% less adhesive force than to non-treated surfaces.

The scientists tested the adhesion of two bacterial strains ( Streptococcus mutans and Streptococcus oralis) that are associated with cavities and one strain that is not (Staphylococcus carnosus). They found that treating the pellets with fluoride made it harder for bacteria to stick to the surface. All of the bacterial strains they tested clung to the fluoride-treated pellets with roughly 50% less adhesive force than to the untreated pellets.

One thing to keep in mind, however, is that the pellets used in these experiments were treated with fluoride only once, whereas our teeth are subject to a lifetime of fluoride exposure–meaning our actual teeth may even be slipperier. Also, it would have been great if the researchers had tested fluoride treatments at different concentrations, such as found in tap water and in dental treatments, to see if using more fluoride would make it even harder for the bacteria to adhere to the pellets and vice versa.

But the question remains: how exactly does fluoride make it harder for bacteria to stick to fluoride-treated teeth. The authors of the study suggest that fluoride treatment increases the negative charge on the surface of teeth, which would repel the negative charge on the surface of bacterial cells. In turn, this would make it easier to dislodge bacteria when you brush your teeth–provided you remember to brush them (yeah, you know who you are).